In general, a thorough test of a high-speed digital system includes a procedure for sampling signals from hardware components of the system (e.g., circuit boards, interconnects, silicon devices, etc.) in order to determine how these components affect signal integrity. To access these signals, an engineer or technician typically connects specialized measurement equipment to the system hardware. In some configurations, such equipment includes a measuring device (e.g., a Time Domain Reflectometer or TDR) that connects to a circuit board which is either (i) part of the high-speed digital system under test, or (ii) a specialized assembly (e.g., a daughter card) that closely integrates with the system under test (e.g., through high-density connectors). Typically, the engineer permanently mounts a coaxial radio frequency (RF) connector to a specialized feature of the circuit board called a signal launch. The engineer can then attach a coaxial cable (e.g., a 50 ohm cable) from the measuring device to the soldered RF connector in order to access signals of the system under test.
One type of permanent mount RF connector, which is hereinafter referred to as an edge mount connector, has soldering posts that attach to an edge mount signal launch along an edge of a special type of circuit board called a microstrip, or microstrip line, which has an exposed signal conductor on one side of a dielectric substrate and an exposed ground conductor on the other side. Both the signal and ground conductors typically run to the edge of the circuit board to form the edge mount signal launch. An engineer typically solders an inner soldering post of the edge mount connector to the signal conductor and solders outer soldering posts to the ground conductor. The engineer can then access signals of the system under test by attaching a coaxial cable from a measuring device to the edge mount connector.
Another type of permanent mount RF connector, which is hereinafter referred to as a posted surface mount connector, has soldering posts that attach to a surface mount signal launch residing on the surface of a circuit board (rather than along an edge) in order to mount to the circuit board perpendicularly relative to the circuit board surface. The soldering posts of the posted surface mount connector are used in a manner similar to the soldering posts of the above-described edge mount connector. That is, an engineer typically solders an inner soldering post of the connector to a signal conductor of the surface mount signal launch and solders outer soldering posts to a ground conductor. The engineer can then access signals of the system under test by attaching a coaxial cable from a measuring device to the posted surface mount connector.
Another type of permanent mount RF connector, which is hereinafter referred to as a flat-faced surface mount connector, has a flat-faced base portion that sits flush on a surface mount signal launch. The flat-faced base portion does not include any protrusions (e.g., soldering posts, tabs, etc.). Rather, the base portion includes (i) a recessed, cylindrically-shaped signal conductor that receives a signal pin of the surface mount signal launch, and (ii) a ground conductor that defines mounting holes. To mount the flat-faced surface mount connector to the signal launch, an engineer typically inserts the signal pin of the signal launch into the signal conductor of the connector base portion, and aligns the mounting holes defined by the ground conductor with matching ground vias of the signal launch. The engineer then inserts screws through (i) the mounting holes of the connector and (ii) the matching ground vias of the signal launch, and fastens nuts on the ends of the screws to hold the connector firmly against the signal launch. The engineer can then attach a coaxial cable from the flat-faced surface mount connector to the measuring device in order to access signals of the system under test using the measuring device.
Most circuit boards, which are configured with a permanently mounted RF connector for external high-speed signal access, have multiple permanently mounted RE connectors to enable an engineers to access many different signals. When the engineers wants to sample a particular signal, the engineers attaches a cable from the measuring equipment to one of the permanently mounted RF connectors. If the engineer wishes to sample a different signal the engineers can detach the cable from that permanently mounted RF connector and attach the cable to another permanently mounted RF connector.
Some measurement equipment does not connect to circuit boards through permanent mount RF connectors. Such equipment typically includes a measuring device, an RF probe and a cable that connects the RF probe to the measuring device. The RF probe alleviates the need for an engineer to permanently mount RF connectors to signal launches of a circuit board. Rather, the engineer simply can temporarily attach the RF probe directly to the signal launch in order to access signals from the circuit board.
One type of RF probe has an elongated shape with a cable connecting portion at one end, and a pointed signal pin at the other end. Near the pointed signal pin is a cylindrical ground conductor. Typically, an engineer attaches a metallic ground extension to the cylindrical ground conductor, and moves the RF probe (i.e., manually manipulates the RF probe) such that (i) the signal pin of the RF probe contacts the signal conductor (e.g., a signal via) of the signal launch, and (ii) the metallic ground extension contacts the ground conductor (e.g., a ground via) of the signal launch. An example of a similar type of RF probe is that manufactured by Tektronix of Beaverton, Oregon.
One type of metallic ground extension has a cuff that fits over the cylindrical ground conductor of the RF probe, and a ground pin that extends from the cuff in the same direction as the signal pin. Another type of metallic ground extension includes a multi-pronged cuff that fits over the cylindrical ground conductor of the RF probe such that multiple prongs of that cuff extend in the same direction as the signal pin. Yet another type of metallic ground extension includes a spring that fits around the cylindrical ground conductor of the RF probe such that a wire end of the spring extends in the same direction as the signal pin. Accordingly, a portion of each type of metallic ground extension can engage a ground via of the signal launch (e.g., the ground via that normally receives a screw to permanently mount an RF connector) as the signal pin engages a signal via of the signal launch.
Unfortunately, there are deficiencies to the above-described conventional approaches to connecting to signal launches of circuit boards. For example, in connection with conventional permanent mount RF connectors, such connectors are typically expensive. Although an engineer typically connects a measuring device to only one or two permanent mount RF connectors on a circuit board at any given time, a circuit board with signal launches typically includes many permanent mount RF connectors for convenience. As such, in order to sample a different signal of the circuit board, the engineer can simply unscrew a coaxial cable from one connector and screw it onto another connector. Moreover, since such connectors are permanently mounted (e.g., soldered or screwed onto the circuit board), it would be a significant burden to require the engineer to unfasten (e.g., unsolder or unscrew) a conventional permanently mounted RF connector from one signal launch of the circuit board and refasten (e.g., solder or screw) that connector to another signal launch each time the engineer samples a different circuit board signal. Accordingly, the use of many expensive RF connectors results in a high cost for sampling signals.
Additionally, some conventional permanent mount RF connectors are not very well impedance-matched with signal launches. For example, in connection with posted surface mount connectors, there are typically large distances between the signal post and the ground posts which form large inductance loops that lower the signal integrity of high-speed signals.
Similarly, some conventional RF probes are not well impedance-matched with signal launches. That is, the conventional RF probe may be well-matched from the measuring device through the cable to the RF probe, but not well-matched with the signal launch. For example, the above-described elongated RF probe can be connected to a signal launch having a central signal launch, an outer set of ground vias which are used as mounting holes (e.g., for solder or screws), and an inner set of ground vias disposed between the central signal launch and the outer set of ground vias. The cuffed or springed ground extensions typically are sized to engage one or more of the outer set of ground vias. Accordingly, the large distances between the pointed signal pin and the connecting portions of the ground extensions of the RF probe form large inductance loops that provide signal distortion. Although the springed ground extension is somewhat flexible, connecting the springed ground extension to one of the inner ground vias while the pointed signal pin of the RF probe engages the signal via of the signal launch requires awkward manipulation of the ground extension (e.g., twisting, bending, extreme stressing, etc.) that can permanently deform or damage the RF probe.
One may consider using some conventional permanent mount RF connectors as probes for forming temporary connections with circuit board signal launches. For example, an edge mount connector with posts could be used to connect with an edge mount signal launch without being permanently fastened to the signal launch (e.g., without being soldered). Unfortunately, the conventional edge mount connector alone would have a tendency to slide around if not somehow fastened to the circuit board. As another example, a conventional posted surface mount connector could be used to connect with a surface mount signal launch without being permanently soldered to the signal launch (e.g., without being soldered). Unfortunately, as described above, the conventional surface mount signal launch forms large inductance loops with the signal launch thus degrading signal integrity. As yet another example, a conventional flat-faced surface mount connector could be used to connect with a surface mount signal launch without being permanently fastened to the signal launch (e.g., without being screwed to the signal launch). Unfortunately, the conventional flat-faced connector alone has a tendency to slide around the signal launch if not screwed to the signal launch since the flat-faced connector sits flush on the signal launch and does not have any protrusions that insert into the signal launch for stability.
In contrast to the above-described conventional permanent mount RF connectors and the above-described conventional RF probe, the invention is directed to techniques for connecting to a signal launch using an RF probe having a flat-faced side with a ground post extending from a non-peripheral region (e.g., an intermediate region between a central region and a peripheral region of the flat-faced side). The use of such a probe alleviates the need to use many RF connectors permanently mounted to signal launches on a circuit board, and thus reduce costs, since the probe of the invention can be temporarily connected to the signal launches without such permanently mounted connectors. Furthermore, the ground post of the invention probe can be configured to avoid large inductance loops thus preserving signal integrity particularly when sampling high-speed signals.
One arrangement of the invention is directed to an RF probe for connecting to a signal launch. The RF probe includes a cabling portion for coupling to a cable, and a base portion attached to the cabling portion. The base portion has a flat-faced side that faces the signal launch when the RF probe connects to the signal launch. The flat-faced side has (i) a central region, (ii) an outer region that extends along a periphery of the flat-faced side, and (iii) an intermediate region disposed between the central region and the outer region. The RF probe further includes a signal post for connecting to a signal conductor of the signal launch and a ground post for connecting to a ground conductor of the signal launch, when the radio frequency probe connects to the signal launch. The signal post extends from the central region of the flat-faced side of the base portion. The ground post extends from the intermediate region of the flat-faced side of the base portion. The distance between the signal and ground posts can be made to form inductance loops which are smaller than those created by the above-described conventional approaches, and thus minimize signal distortion.
The signal and ground posts enable a user to hold the RF probe in place relative to the signal launch. In particular, the user can press the RF probe against the signal launch such that interference between the posts and vias of the signal launch prevents the RF probe from sliding around.
In one arrangement, the ground post is disposed in a fixed position on the flat-faced side of the base portion. In this arrangement, the base portion of the RF probe includes a ground conductor, and the ground post preferably connects to the ground conductor of the base portion at a solder joint.
In one arrangement, the base portion includes a ground conductor that defines mounting holes. In this arrangement, the ground post preferably extends from a location of the intermediate region that is between the signal post and one of the mounting holes. Accordingly, a flat-faced permanent mount RF connector (e.g., the earlier described conventional surface mount RF connector with mounting holes) can be used as the cabling and base portions of the probe.
In one arrangement, the RF probe has multiple ground posts. That is, in addition to the ground post, the RF probe includes another ground post for connecting to another ground conductor of the signal launch when the radio frequency probe connects to the signal launch. The other ground post extends from the intermediate region of the flat-faced side of the base portion. This arrangement provides multiple pathways for a return ground signal for more uniform current distribution.
In another arrangement, the RF probe has only one ground post. That is, the ground post of the RF probe is a single exclusive ground post. This arrangement limits the amount of conductive material in the RF probe thus providing less conductive material for less capacitance. Furthermore, this arrangement is simpler to make than the arrangement with multiple ground posts (e.g., less soldering of ground posts to the base portion).
In one arrangement, the base portion includes a ground conductor that electrically connects with the ground post. Preferably, the RF probe further includes an insulator that covers at least part of the ground conductor of the base portion. Accordingly, a user can easily hold and move the RF probe without inadvertently touching the ground conductor. In particular, the user can easily maneuver and manipulate the RF probe (e.g., move the RF probe among multiple signal launches).
The features of the invention, as described above, may be employed in testing systems, devices and methods and other computer-related assemblies such as those of Teradyne Corporation of Boston, Massachusetts.